The invention relates to a new process for embedding rare earth fluorides into silica glasses. The new embedded glasses are useful as optical amplifiers for telecommunications.
In recent years there has been an explosive deployment of optical amplifiers, particularly erbium doped fiber amplifiers, in optical telecommunication systems. This phenomenon is due in part to the well-recognized advantages that these types of devices have over repeater type amplification schemes. For example, the erbium doped fiber amplifier (EDFA), which conveniently operates in the preferred 1550 nm, or so-called third telecommunication spectral window, has high polarization-non-sensitive gain, low cross talk between signals at different wavelengths, good saturation output power, and a noise figure close to the fundamental quantum limit. The excellent noise characteristics potentially allow hundreds of amplifiers to be incorporated along the length of a optical fiber link, which could span thousands of kilometers. Optical amplifiers, particularly EDFAs, in contrast to electronic repeaters, are also transparent to data rate, signal format and wavelength over a limited range, making them especially useful for wavelength multiplexed communication systems that simultaneously transmit a large number of signals using different wavelength bands for each signal.
Currently, germania-doped silica and heavy metal fluoride (such as ZBLAN) glasses are used as hosts for rare earth ions, such as erbium and praseodymium, to make fiber amplifiers. Silica base glasses are chemically and mechanically stable. They are both relatively easy to fabricate and fuse with germania-doped silicate fibers. Existing silicate glasses, however, are inefficient for infrared upconversion because of their large phonon energy. On the other hand, fluoride glasses have low phonon energy, but are very difficult to fiberize. Fluoride glasses are also difficult to fuse with silicate fibers.
Transparent oxyfluoride glass-ceramics, which are comprised of fluoride microcrystals in a glassy matrix, offer unique properties of high chemical durability and a low phonon energy environment for rare earth ions dopants. These glass-ceramics are made by melting oxides and fluorides of cation components to form an oxyfluoride glass. Typically, the glass-ceramic is made in a two-step ceramming process. A first heat treatment forms nuclei in the glass, from where in a second heat treatment the fluoride micro-crystals grow. Because these transparent oxyfluoride glass-ceramics generally have lower melting temperatures, higher refractive indexes and higher thermal expansion coefficients than silicate glasses, they may pose a challenge to making fiber amplifiers that are covered with a pure silica overcladding.
Accordingly, there continues to be a need for a glass-ceramic material that is CTE-matched for use in optical amplifiers and methods for making such products. In particular, there is a need for new ways for embedding rare earth fluorides into glasses suitable for making optical amplifiers.
Briefly, the present invention relates to a silica-based glass containing rare earth-fluoride crystals. In another aspect, the invention relates to a method for embedding rare earth fluorides into silica-based (or germania-doped silicate) glasses by means of solution chemistry. The term xe2x80x9csilica-based glassxe2x80x9d or xe2x80x9csilicate glassxe2x80x9d as used herein refers to any glass having silica and/or germania-doped silica as the predominant component of greater than or equal to about 70% by weight.
The rare-earth-ion-doped glass preforms, according to the present invention, have a fundamental composition, on an oxide-basis in terms of weight percent, comprising (or consisting essentially of):
wherein X and Y are integers not equal to 0. The amounts of SiO2+GeO2 are fully interchangeable. Each componentxe2x80x94SiO2 or GeO2xe2x80x94could range from 0% to about 99%, as long as the total sum of SiO2+GeO2 is between about 85% and 99%. The sums of Al2O3 and Ga2O3 also are interchangeable. R is an alkali or alkaline-earth ion such as Na, K, Li, Ca, or Mg. RE is a rare-earth ion.